Why Does Water Put out a Fire?

When a fireman squirts water on the burning wall of a building, he is putting the fire out in two ways. The water cools the burning material below its kindling point (the temperature below which it will not burn). At the same time, the heat of the fire turns the water to steam.

The expanding steam smothers the fire by shutting out the oxygen needed by the flame, causing the fire to die out. Fire extinguishers usually contain chemicals that form a gas that will not burn. The gas spreads over the burning fuel, shuts out the air, and smothers the fire.

Water turns out to be an excellent liquid for lowering the temperature of a fire. It doesn’t combusts like alcohol or gasoline, which is kind of important. But even in comparison to other non-combustible liquids, water has both a high specific heat and a high heat of vaporization.

The specific heat is the amount of energy that must be absorbed to raise the temperature of the water, and the heat of vaporization is the amount of heat that must be absorbed to evaporate the water.

Thus, water does a really good job of absorbing energy as its temperature is raised and as it evaporates, which makes it very efficient in putting out fires by lowering the temperature enough to interrupt the chain reaction.